The present invention provides a process for producing particles suitable for use as abrasives in chemical-mechanical polishing slurries and particles formed according to the process.
Chemical-mechanical polishing (CMP) slurries are used, for example, to planarize surfaces during the fabrication of semiconductor chips and the like. CMP slurries typically include chemical etching agents and abrasive particles dispersed in a liquid carrier. The abrasive particles perform a grinding function when pressed against the surface being polished using a polishing pad.
It is well known that the size, composition, and morphology of the abrasive particles used in a CMP slurry can have a profound effect on the polishing rate. Over the years, CMP slurries have been formulated using abrasive particles formed of, for example, alumina (Al2O3), ceric oxide (CeO2), iron oxide (Fe2O3), silica (SiO2), silicon carbide (SiC), silicon nitride (Si3N4), tin oxide (SnO2), titania (TiO2), titanium carbide (TiC), tungstic oxide (WO3), yttria (Y2O3), zirconia (ZrO2), and combinations thereof. Of these oxides, ceric oxide (CeO2) is the most efficient abrasive in CMP slurries for planarizing silicon dioxide insulating layers in semiconductors because of its high polishing activity.
Calcination is by far the most common method of producing abrasive particles for use in CMP slurries. During the calcination process, precursors such as carbonates, oxalates, nitrates, and sulphates, are converted into their corresponding oxides. After the calcination process is complete, the resulting oxides must be milled to obtain particle sizes and distributions that are sufficiently small to prevent scratching.
The calcination process, although widely used, does present certain disadvantages. For example, it tends to be energy intensive and thus relatively expensive. Toxic and/or corrosive gaseous byproducts can be produced during calcination. In addition, it is very difficult to avoid the introduction of contaminants during the calcination and subsequent milling processes. Finally, it is difficult to obtain a narrow distribution of appropriately sized abrasive particles.
It is well known that CMP slurries containing contaminants and/or over-sized abrasive particles can result in undesirable surface scratching during polishing. While this is less critical for coarse polishing processes, in the production of critical optical surfaces, semiconductor wafers, and integrated circuits, defect-free surfaces are required. This is achievable only when the abrasive particles are kept below about 1.0 xcexcm in diameter and the CMP slurry is free of contaminants. The production of abrasive particles meeting these requirements by conventional calcination and milling techniques is extremely difficult and often not economically feasible.
An alternative method of forming abrasive particles for use in CMP slurries is hydrothermal synthesis, which is also known as hydrothermal treatment. In this process, basic aqueous solutions of metal salts are held at elevated temperatures and pressures for varying periods of time to produce small particles of solid oxide suspended in solution. A methods of producing ceric oxide (CeO2) particles via hydrothermal treatment is disclosed, for example, in Wang, U.S. Pat. No. 5,389,352.
The production of abrasive particles by hydrothermal treatment provides several advantages over the calcination/milling process. Unfortunately, however, abrasive particles formed by conventional hydrothermal treatment processes tend not to provide desired high polishing rates.
The present invention provides a process for producing particles suitable for use as abrasives in chemical-mechanical polishing slurries. The process comprises adding a crystallization promoter such as titanium(IV) isopropoxide to an aqueous cerium salt solution, adjusting the pH to higher than 7.0 using one or more bases, and subjecting the solution to hydrothermal treatment at a temperature of from about 90xc2x0 C. to about 500xc2x0 C. to produce particles. Although the precise mechanism is not yet precisely understood, the presence of a crystallization promoter in the solution during hydrothermal treatment results in the formation of particles with larger than expected crystallite sizes. Particles formed in this manner polish surfaces at a much higher rate than particles formed by conventional hydrothermal processes.
The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.